This study was designed to evaluate effects of aging on glycogen synthase activity in rat skeletal muscle. Total enzyme activity was shown to be significantly, (p less than .001) lower in tensor fascia latae, biceps femoris, and soleus muscle obtained from 24-month-old compared with 2-month-old rats. Similarly, values for the active form of enzyme were significantly lower, (p less than .001) in all three muscle types of 24-month-old compared with 2-month-old rats. This age-related decline in glycogen synthase activity was not due to a reduction in the affinity of the enzyme for its activator (glucose-6-phosphate) and was independent of the concentration of substrate (UDP-glucose) in the assay system. Because similar age-related changes were seen when enzyme activity was expressed per milligram of muscle protein or per gram of muscle tissue, the fall in enzyme activity was not a simple function of an age-related decline in muscle mass. Glycogen levels also were reduced significantly in tensor fascia latae, biceps femoris, and soleus of 24-month-old rats compared with 2-month-old rats, p less than .001. These results document an age-related change in a key enzyme regulating glycogen metabolism in muscle.